In general, when a semiconductor wafer is to be processed or inspected, the orientation of the wafer must be aligned to take account of the directionality of the crystal of the wafer. For that reason, a simple method is known of forming a linear cut-out portion called an orientation flat in the wafer, to be used when the orientation flat is aligned by rollers.
This use of rollers in this manner to position the orientation flat enables wafers to be aligned in a batch while still accommodated within a cassette, and such a mechanism is used at a heat treatment station that performs a heat treatment on a batch of wafers, for example, with a cassette placed vertically to position the wafers. When a process such as ion implantation or patterning is performed on wafers, however, the wafers must be accurately placed with respect to the ion implantation region or exposure region, and thus it is necessary to center the wafers as well.
Techniques are known in the prior art as methods of positioning the orientation (the direction of the orientation flats) and the centers of the wafers, such as a technique disclosed in Japanese Patent Application Laid-Open No. 60-85536, of aligning the orientation of a wafer by a turntable and also moving that turntable in the X and Y directions to position the center thereof in a general manner, or a technique disclosed in Japanese Patent Publication No. 1-28503 of orientating a wafer on a stage by rollers and then moving that stage in the X and Y directions.
Since it is wasteful to form an orientation flat in a wafer and thus cut out a wide area thereof, a method that has recently been investigated involves forming a semicircular cut-out portion called a notch instead of a orientation flat in the peripheral edge of a wafer, and thus a mechanism that uses rollers to position wafers within a cassette cannot be applied thereto without changes.
In addition, a sealable wafer cassette has been investigated as means of suppressing contamination of the wafers by particles, but in such a case it is not possible to position the wafers within the cassette. The wafers have to be removed from the cassette for positioning, but since a plurality of wafers, such as five wafers, have to be moved at a time into a wafer boat at a heat treatment station that performs a heat treatment on a batch of wafers, for example, the previously described method of placing wafers one-by-one on a turntable would reduce the throughput.
The previously mentioned method that uses a turntable to center a wafer necessitates X-direction and Y-direction drive mechanisms and moreover the amounts of drive thereof must be accurate, so that the cost is increased by this increase in mechanical complexity. A sealed type of wafer cassette that is designed to suppress the contamination of wafers by particles is provided with a main cassette body, which accommodates a number of wafers (such as 13 wafers), and a lid member for hermetically closing a wafer transfer port formed in the main cassette body with an O-ring therebetween. The configuration is such that the lid member is locked by, for example, the insertion of keys into the lid member, whereby it is fixed to the main cassette body.
To reduce the contamination of the wafers by particles in this case, it is preferable to partition a cassette mounting region for placing the wafer cassette from a wafer transportation region, and keep the degree of cleanliness of the cassette mounting region higher than that of the wafer transportation region. The cassette mounting region will be referred to as a first environment and the wafer transportation region will be referred to as a second environment. In one method of increasing the cleanliness of the first environment that can be considered, the first environment and the second environment are separated by a partitioning wall, the wafer cassette is attached from the first environment side to an aperture formed in the partitioning wall, and the wafers within the wafer cassette are removed toward the second environment side.
With the above-described configuration for attaching a wafer cassette, the various components are constructed in such a manner that a gap on the order of approximately 2 mm is formed between the wafer cassette and the partitioning wall at the peripheral edge portion of the aperture when the wafer cassette has been attached to the aperture in the partitioning wall. Since the pressure in the second environment is set to be higher than that in the first environment, clean air always flows into the first environment through this gap, and thus particles are prevented from penetrating from the first environment side into the second environment side.
However, the air flowing through this gap forms vortices. In the vicinity of the surfaces that define the gap, such as those of the partitioning wall and the wafer cassette, the air is extremely gentle with a velocity close to zero. Thus the air on the first environment side is entrained into the vortices of the air flowing from the second environment, so that it flows along the above surfaces into the second environment side. This causes particles to penetrate into the second environment together with the air and thus, although great care has been taken to use sealed cassettes to prevent contamination of the wafers by particles, that intention is counteracted. In addition, if the region used for transferring wafers into a wafer boat is made to be a nitrogen environment, for example, it becomes difficult to manage the oxygen density within a processing region of an apparatus designed to prevent the growth of natural oxides.
A sealed type of wafer cassette makes it impossible to position the wafers in the cassette, thus making it necessary to position the wafers after they have been removed from the wafer cassette. The exchange of wafer cassettes involves attaching the lid member to the current wafer cassette, removing the wafer cassette from the partitioning wall, and attaching a new wafer cassette to the aperture in the partitioning wall. The removal and attachment of the lid member and the removal and attachment of the wafer cassettes themselves are time-consuming processes, so that the exchange of wafer cassettes takes a long time. Therefore, since the incorporation of a system that uses a sealed type of wafer cassette makes it necessary to add the time required for positioning the wafers and the time required for exchanging the wafer cassettes, there are demands for the investigation of techniques preventing any deterioration in throughput.